1. Field of the Invention
This invention relates to a superconducting device, and more particularly to an anisotropic superconducting device that uses an oxide superconductor having a high superconduction critical temperature that exceeds the temperature of liquid nitrogen.
2. Discussion of the Background
Advances in thin film fabrication technology and high precision microfabrication technology have produced a rapid increase in research on electronic devices that use the class of oxide superconductors referred to as high-temperature superconductors, meaning superconductors that have a high superconduction critical temperature. However, there is still a lack of knowledge when it comes to computer devices, especially electronic devices used mainly for digital applications. In the case of junction type or multi-layer type devices which consist of a barrier layer formed between two superconductor layers, a stable, highly reliable device, such as a lead or niobium based Josephson switching device for use at ultralow temperatures, has not been obtained.
As junction devices that use high-temperature superconductors, there can be considered a superconductor-normal (i.e., ordinary metal) conductor-superconductor (SNS) junction device or a superconductor-insulator-superconductor (SIS) junction device. However, a problem that does not arise in the case of a conventional metallic superconductor but does arise in the case of high-temperature superconductors is that of the anisotropy of the superconductive properties (such as coherence length, magnetic field penetration depth and energy gap) produced by the crystal orientation. For example, the coherence length of a film of YBaCuO, a high-temperature oxide superconductor, is several tenths of a nanometer along the c axis and several nanometers in the Cu-O plane along which most of the superconducting electrons pass. Thus, compared to a metallic superconductor the coherence length of a high-temperature superconductor is shorter and anisotropic, and according to the accepted thinking this forms an obstacle, especially with respect to the realization of an SIS device.
A prototype SNS junction device has been fabricated consisting of upper and lower layers of YBaCuO superconductor separated by a barrier layer of PrBaCuO, which even at ultralow temperatures does not become superconductive but instead retains its semiconductive properties. While the device uses a-axis-oriented YBaCuO films in which the Cu-O planes are perpendicular to the plane of the junction, a-axis YBaCuO films have no anisotropy in the plane of the junction.
While a Josephson switching device which operates in latching mode at ultralow temperatures, such an a-axis YBaCuO/PrBaCuO/YBaCuO junction device operates in non-latching mode that can be observed in Josephson weak link devices similar to microbridge devices (that is, in principle without hysteresis characteristics), and as such can be used as an electronic function device. However, even though it may have the advantages that the use of a high-temperature superconductor provides, such as a lighter cooling burden, it does not have any major advantage over existing ultralow temperature Josephson devices in terms of operation or properties, and has drawbacks such as the magnitude of absolute critical current values.